1. Field of the Invention
The invention relates to an expansion card which is connected to a predetermined bus of a computer apparatus and executes a predetermined expanding function process synchronously with a clock which is supplied, an access control method for use with the expansion card, and a memory medium in which a program that can be read out by a computer has been stored.
2. Related Background Art
Hitherto, an expansion bus to expand a function is provided for a computer such as a personal computer or the like. A card which is attached to the expansion bus is called an expansion card.
Such an expansion card transmits and receives data to/from a computer main body via signal lines (mainly an address/data line, a control signal line, and the like) on the expansion bus, thereby providing an expanding function to the computer.
As a data transmission system using the expansion bus, there are an asynchronous data transmission system and a synchronous data transmission system. The asynchronous transmission system is a system in which data is transmitted by a hand-shake by using a control signal without using a common clock signal at the time of the data transmission and is used in a relatively low speed expansion bus.
On the other hand, the synchronous transmission system is a system in which data is transmitted by using a common transmission clock signal and can perform a data transmission at a speed higher than that of the asynchronous transmission system.
As for the generation of a fundamental operation clock of the expansion card which is connected to the expansion bus of the synchronous transmission system, two kinds of systems are considered. One of them is a system in which the operation clock is generated by using a data transmission clock signal which is supplied from a computer main body via the expansion bus and the whole circuit operates. As shown in a timing chart of FIG. 7, both of the above clocks are synchronized. According to the other system, by providing a substantially independent operation clock generator onto the expansion card, the whole circuit of the card operates a synchronously with the data transmission clock as shown in a timing chart of FIG. 8.
However, in the expansion card in which the whole circuit operates on the basis of the data transmission clock which is supplied from the computer main body, in the case where the expansion bus is an expansion bus such that a frequency of a clock signal can arbitrarily be set in the computer main body as in an expansion bus called, for example, a PCI, an operating speed of the expansion card is influenced by a frequency of the transmission clock of the expansion bus, so that there is a problem that when the frequency of the transmission clock decreases, the performance of the expansion card also deteriorates.
In an expansion card such that the whole circuit operates on the basis of substantially independent clock signals, when a difference between the frequencies and phases of both of the clock signals is absorbed, an overhead occurs and the transmitting speed decreases and there is also a problem in that a FIFO memory is necessary for matching the synchronization or the like and requiring a large-scale matching circuit or the like.
The invention is made to solve the above problems, and it is an object of the invention to provide an expansion card in which by variably setting a frequency of a reference clock in accordance with a fluctuation of a data transmission clock which is supplied through an expansion bus, a data process of high performance can be realized while generating the reference clock of a predetermined frequency without being influenced by a frequency fluctuation of the data transmission clock which is supplied through the expansion bus, and a memory access based on the reference clock of the predetermined frequency which is not influenced by the frequency fluctuation of the data transmission clock which is supplied via the expansion bus can be realized, and to also provide an access control method of the expansion card and a memory medium in which a program that can be read out by a computer has been stored.